Implantable cardiac rhythm management devices (ICRMDs) sense the heart's electrical activity and, if an arrhythmia is detected, deliver therapy to terminate it. Many patients with ICRMDs are at particular risk for myocardial ischemia, which has been reported to trigger arrhythmias (20-50% of ventricular tachyarrythmias).
Presently, to assess the presence or absence of ischemnia in patients, cardiac electrophysiologists (EPs) perform 12-lead body-surface electrocardiograms (ECGs) or 24-hour Holter monitoring. Myocardial ischemia has been known for years by EPs to be manifested on ECGs as ST-segment deviation (STD). Unfortunately, 12-lead ECGs are obtained over a short duration (&lt;1 hour), and generally detect only chronic levels of ischemia. Holter recordings are more apt to detect transient ischemic events, but the 24-hour recording periods are still only snapshots of the patient's ischemic condition.
Recently, investigations have shown that similar STD changes occur in the electrograms from intra- and extracardiac signals. Other morphological features of electrocardiograms have also been reported to be representative of ischemia, including ST-segment-duration decrease, R-wave-duration increase, R-wave-amplitude decrease, T-wave duration/amplitude change, change in the rate of rise/fall of the T-wave, decrease in T-wave uniformity, and change in position of the J point.
Implantable medical devices have been suggested as being useful for signal analysis of the electrical potential curve sensed from the heart. One suggested approach has been to sense cardiac signals with an implantable medical device and then compare them to a template signal stored in the medical device. Based on this comparison, an assessment as to whether myocardial ischemia is present can be made. Using the stored template signal, however, may not yield the most accurate and reliable way of detecting the presence of myocardial ischemia. Once the template signal has been programmed, it remains constant. In contrast, the cardiac signals being sensed by an implantable device changes over time as a result of changes to the tissue that surrounds the implanted electrodes. As the tissues change, so do the signals the electrodes are able to sense. So, the cardiac signal sensed by the implantable medical device begins to change relative the stored template signal. As this change takes place, the template signal becomes less and less useful in identifying myocardial ischemia.
Additionally, the entire baseline value of the sensed cardiac signal may change over time. Gradual changes in the baseline value of a cardiac signal can indicate a change in cardiac condition which a physician may wish to investigate more closely. Thus, a need still exists for improvements in acquiring measurements and comparing those measurement in an effort to identify myocardial ischemia.